This invention relates to a plastic refractory especially suitable for pneumatic gunning. It relates more specifically to a plastic refractorY which may be gunned more efficiently because less rebound occurs and because the flow of the granulated refractory through the gunning apparatus is improved.
A plastic refractory is a moist, pliable mixture of aggregates and binders which when applied to a furnace wall or the like and fired in place forms a hard, monolithic, refractory lining for the substrate. Plastic refractory compositions are manufactured in granular form and in slab form. Both forms have been and still are placed by ramming the plastic masses onto the substrate to knead and knit them together and form a monolithic lining. Ramming is labor intensive and much care must be taken to avoid lamination of the plastic masses which would shorten the useful life of the refractory lining.
The gunning of plastic refractories has the advantages of being potentially less labor intensive and of forming non-laminar linings. The potential for savings in labor costs is reduced in some circumstances by the large amounts of refractory material that rebounds from the surface being covered. Also, the larger pieces of aggregate may be lost by rebound while the smaller ones remain, resulting in the emplacement of a refractory widely different in composition than the starting material. In some cases the rebounded material can be recovered and reworked into a pliable mass but in others it is not usable because of contamination. In either circumstance, time is lost in recovering the rebound and in gunning for a longer period to build up the desired thickness. Moreover, the very nature of the plastic refractory makes it susceptible to compaction during storage and even during shipment. Thus, a plastic refractory which has been granulated for application by the gunning technique may not be in gunnable condition at the site because of compaction. Also, attempts to minimize the amount of rebound by increasing the amount of water in the refractory mixture have often had the undesirable effect of causing the mixture to cling to the internal surfaces of the delivery hoses and pumps and eventually clog the apparatus, thereby causing work stoppages. Adding the water as a separate stream into a dry refractory mix as it passes through the gun nozzle calls for a considerable amount of skill to maintain the proper plasticity so that rebound is minimized and laminations are avoided.
Greener teaches in U.S. Pat. No. 4,535,001 that conventional plastic refractory materials can be gunned without the need to add water or modify the composition by the simple expedient of gunning the refractory material immediately after the granulation thereof. To do so requires that special equipment such as the granulator and conveyor taught by Greener must be at the job site. The cost of building, maintaining, and transporting such equipment may offset the advantage of being able to use conventional ramming-type plastic refractory material instead of specially formulated materials.